It's probably worth noting that changing:

from email.mime.text import MIMEText

m = MIMEText('foo')
m['subject']='something long'

...to:

from email.header import Header
m = MIMEText('foo')
m['subject']=Header('something long')

...will do folding without the \t problem, even in Python 2.6

I guess the moral of the story is that all headers should really be
header objects. I think Barry has some ideas on that ;-)